1. Field of the Invention
The present invention relates to a display control apparatus which includes a flat panel display unit such as a plasma display unit or a liquid crystal display unit as a standard I/O device and can be arbitrarily connected to a CRT display unit, and can be suitable for use in a computer system such as a personal computer and a personal workstation.
2. Description of the Related Art
Liquid crystal display and plasma displays have been used as display units for conventional lap-top type personal computers and personal workstations. However, strong demand has arisen for running an application program created for a CRT display unit in the above computer so as to utilize a wealth of all previous software. For this reason, the above computer equipped with a plasma display unit as a standard I/O device is arranged to be connectable to a CRT display unit as an optional I/O device.
A CRT display unit multigradation palette (CRT palette) is separated from a plasma display unit gradation designation (4 through 16 gradation levels) palette (PDP palette), and different palette values are set therein.
For this reason, when a CRT application program runs to dynamically update the CRT palette, the palette value of the PDP palette is not changed even if the CRT palette value is changed.
Similarly, in a system to which a CRT color display unit can be arbitrarily connected, different palette values are set in a color palette for designating display colors of the CRT color display unit and the PDP palette. Even if the palette value of the CRT palette is arbitrarily changed in accordance with a display color designated by CRT application software, the palette value of the PDP palette is not changed. For this reason, although a change in gradation or hue can be identified on the CRT display screen, the change cannot be reflected on the PDP, resulting in inconvenience.
In order to solve the above problem, Hiroki ZENDA (U.S. Ser. No. 406,066 titled "DISPLAY CONTROL APPARATUS FOR CONVERTING COLOR/MONOCHROMATIC CRT GRADATION INTO PDP GRADATION", filed on Sep. 12, 1989, and assigned by the same assignee of this application) discloses a technology where plasma display unit gradation data is generated from CRT display unit gradation data set in a CRT palette and the plasma display unit gradation data is output to the PDP palette.
FIG. 1 is a block diagram showing a detailed arrangement of a high-resolution display system (HRGS: High Resolution Graphic Subsystem) having a display control function for driving the CRT display at a high resolution (720 dots in the horizontal direction) with multilevel gradation (64 gradation levels) and a display control function for simultaneously driving CRT and plasma display units (in this case, each display unit has a horizontal display resolution of 640 dots). A CRT display unit can be arbitrarily connected to the HRGS 30 through a connector not shown.
An element 31 is a CRT display controller (CRT-CNT) for driving a CRT display unit 51 at the high resolution (720 dots) with multilevel gradation (64 gradation levels). The CRT-CNT 31 includes a decoder 311. The decoder 311 decodes a port address of a CRT palette 343 and outputs a write signal to the CRT palette 343.
A display controller 32 has a bus interface function for exchanging various display control data with a CPU through a system bus 16 and a function for controlling the display operation of a plasma display unit (PDP) 40 and is constituted by one chip of, e.g., a programmable logic array (PLA). The display controller 32 includes an arithmetic unit 325. The arithmetic unit 325 comprises, e.g., a programmable logic array and performs a predetermined arithmetic operation of palette data supplied from a CRT palette 343 through a data bus (SD7-SD0) 62 to output 39 gradation parameters. A conversion table 326 is arranged to output 16 gradation data corresponding to 39 gradation parameters.
A CRT-CNT reads out display data (since a color CRT is used in this embodiment, 16 color data are read out) from a VRAM 33, and the readout data are written in the CRT palette 343 in a CRT display data generator 34 through an internal bus (VD7-VD0). The color CRT palette 343 converts 16 color data into 262,141 color tone data (=64 colors.times.(R, G, and B components)). The 262,144 color tone data are converted into analog display data by a D/A converter 342, and analog display data are output to a color CRT display unit 51. As a result, image data represented by color information designated by the CRT palette 343 is displayed on the color CRT display unit 51.
The 64 data of each of the R, G, and B components are supplied to the arithmetic unit 325 in the display controller 32 through the data bus (SD7-SD0). The arithmetic unit 325 calculates gradation parameters on the basis of equations (1) and (2).
A gradation intensity is calculated by equation (1): EQU Gradation Intensity=(3.times.red data)+(6.times.green data)+(1.times.blue data) (1)
The gradation parameter is calculated by equation (2) using the above gradation intensity:
Gradation Parameter=(gradation intensity)/16 (the decimal part is rounded off)
The 262,144 color tone data supplied from the color CRT palette 343 are converted into 39 gradation parameters by the arithmetic unit 325. These 39 gradation parameters are input to the color conversion table 326.
The conversion table 326 is arranged, as shown in FIG. 2. Addresses "00" (hexadecimal notation) through "27" correspond to gradation parameters 0 through 39, respectively.
The color conversion table 326 outputs gradation data (gradation level 0 through gradation level 15) corresponding to the input gradation parameters to the PDP palette 322. As a result, a PDP 40 performs a display in accordance with gradation data (one of gradation level 0 to gradation level 15) set in the PDP palette 322.
FIG. 3 shows a color-gradation conversion characteristic where the gradation parameter and the gradation intensity are given in abscissa and in ordinate, respectively. As shown in FIG. 3, the conversion characteristic is roughly linear. Therefore, luminance level of color data and gradation level are approximately proportional to each other. In other words, a bright color is converted into a bright gradation, and a dark color is converted into a dark gradation. For this reason, if the bright colors are superimposed on each other, e.g., a bright white is superimposed on a bright yellow background, there is little difference in the gradation level, and this makes it difficult to differentiate between these colors visually.